The present invention relates to a circuit arrangement for compensating for the influence of temperature on coil quality, the circuit arrangement having a first coil with N1 turns, which contains a first inductor and a first ohmic resistor, and having a second coil with N2 turns, which is magnetically coupled and connected in parallel with the first coil and contains a second inductor and a second ohmic resistor.
A circuit arrangement disclosed by the reference DE 35 26 560 A1 deals with a problem of temperature dependence for an inductive sensor, which is operated with an inductor to measure distances or displacement positions. The dependency on temperature, which is disturbing in this case, comes about because of the temperature coefficient response to temperature changes of the coil wire. Therefore, the inductive sensor is designed in a way that allows the temperature dependence of the measured values to be reduced. To this end, a magnetically coupled auxiliary winding is allocated to the inductor and an NTC (negative temperature coefficient) thermistor is connected up to this auxiliary winding. In this case, the auxiliary winding is electrically isolated and shunted out via the NTC thermistor. Accordingly, relatively high currents flow in the auxiliary winding, which results in correspondingly high losses.
The German Published Patent Application 20 10 134 discloses a circuit arrangement for inductively measuring the position or displacement of a test object, having a transformer, whereby the test object lies between the primary and secondary winding of the transformer. With this configuration, the position or change in position of the test object is supposed to be measured. The intention is to reduce or even eliminate measuring errors due to variations in the temperature of the test object and the resultant highly temperature-dependent magnetic properties. To achieve this, an additional element, which compensates for the temperature dependence of the permeability of the test object, is allocated to the primary winding of the transformer and is included in its electric circuit. The following solutions are proposed for the compensating element. The element is, for example, an additional and temperature-dependent inductor that is connected in series with the primary winding. However, it can also be conceived as a resistor connected in series with the primary winding or as a resistor bank having positive temperature coefficients. A further development of the element consists of connecting a resistor or a resistor bank having negative temperature coefficients parallel to the primary winding. In the case of the described circuit arrangement, emphasis is placed on the temperature dependence of the permeability of the test object when a transformer is used.
The European patent document EP O 070 796 introduces a method for compensating for the temperature dependence of the oscillatory amplitude of a resonant circuit excited by a generator. From the coil of the resonant circuit made out of litz wire (i.e., stranded or flexible wire), one litz wire is separated from the other at one connection of the coil and is brought out to a separate terminal, so that the coil situated between these two connections can be regarded as doubly wound. The combined effect of the inductances of both windings is neutralized in this case, and the connection to the coil of one single litz wire makes it possible to determine the resistance of the conductor material of this coil. A constant a.c. current source, with which the resonant circuit is excited to its resonant frequency, is connected up between the two named connections. A voltage, which is proportional to the resistance of the determined conductor material of the single-wire coil, is injected through the resonant circuit and, accordingly, becomes greater when the temperature rises and is reversed when contrary temperature conditions prevail. The temperature-dependent quality of the resonant circuit can be compensated for through the a.c. current source. However, it is a costly closed-loop control process to compensate by means of an a.c. current source. In this process, the resistance of the single-wire coil must first be measured and, in dependence upon this, the resonant-circuit voltage must be controlled to obtain, for example, a certain quality of the resonant circuit independently of temperature. Besides the expenditure of time and energy that this solution entails, the losses that occur are problematic in some applications when the coil or the resonant circuit are used in units with the lowest possible required power.